1. Field of the Invention
The invention is directed to an improved method of making and using copper-based single crystal shape memory alloys (SMAs). In particular, this invention is directed to SMAs of CuAlNi that are biocompatible.
2. Background
Certain SMAs are used in medical devices, including implants. In particular, nickel titanium SMAs such as “Nitinol” have become widely known as a biocompatible shape memory alloy. Nitinol is a compound of nickel and titanium, and has become used extensively in medical and similar applications because it is more flexible than steel. Another class of SMAs, the copper-aluminum-nickel superelastic alloys also exhibit large shape recovery and would be useful in medical devices were it not for the widespread belief that their large copper content is cytotoxic, and would result in deleterious interaction with living tissue, particularly in long-term embodiments.
SMAs including copper are believed to be cytotoxic because copper is widely accepted as a cytotoxic metal. Copper is among the more frequently reported metals with which patients are poisoned, and routinely ranks third (behind lead and arsenic) in non-medicinal metal exposures reported to US Poison Control Centers. See, e.g., Goldfrank's Toxicologic Emergencies (7th Edition), Chapter 82C, “Copper,” by Lewis S. Nelson (2002: McGraw-Hill).
The cytotoxicity of copper-based shape memory alloys, and particularly single-crystal copper-based shape memory alloys is well documented. The prior art generally teaches away from using exposed single-crystal Cu-based alloys because of this presumed cytotoxicity. For example, Yahia et al. (Yahia, Manceur, and Chaffraix, “Bioperformance of shape memory alloy single crystals”, Bio-Medical Materials and Engineering 16:101-118 (2006)) discusses the presumed cytotoxicity of copper-based single-crystal alloys. Even as recently as 2008, copper-based single-crystal alloys are presumed to be cytotoxic. For example, Creuziger and Crone, (“Initial transformation around a notch tip in CuAlNi: Experiment and modeling,” Acta Materialia, (2008) 56:518-526).
However, copper-based SMAs may be extremely useful. Superelastic single crystal CuAlNi is extraordinarily flexible, even compared to other SMAs (e.g., Ti-Ni alloys and Fe-based alloys). In particular, single crystal CuAlNi alloys may have properties that are highly desirable. For example, a single crystal CuAlNi material may have a strain-recovery that is nearly 10percent strain, which can be described as ‘hyperelastic’ (hyperelastic behavior is described in U.S. patent application Ser. No. 10/588,413, herein incorporated by reference in its entirety). Thus, while shape memory alloys transform from one solid crystal structure to another, and are capable of energy storage at greater densities than elastic materials, in hyperelastic transformations, the energy is absorbed and released at nearly constant force, so that constant acceleration is attainable. Many medical procedures would benefit from improved flexibility, for example, archwires for orthodontistry, guidewires for catheters, and clot retrievers for intracranial and cardiovascular intervention. However, each of these applications requires material that can be exposed to tissues and/or the blood stream without causing toxic damage.
To date, however, virtually nothing is known about the biocompatibility of copper-based SMAs, beyond the assumption in the art that such materials are cytotoxic because of their high copper content, making them unsuitable for biological (e.g., implanted or chronic) use.
We show here that single-crystal copper-aluminum-nickel SMAs may be prepared so that they are biocompatible. Results of MEM elution cell cytotoxicity, ISO intramuscular implant, and hemo-compatibility tests were performed to show that CuAlNi alloys can be fully biocompatible. Copper-aluminum-nickel (or copper-aluminum manganese or beryllium) may be made biocompatible by the formation of a durable oxide surface layer analogous to the titanium oxide layer that inhibits body fluid reaction to titanium nickel alloys. The oxide layer may be made durable and capable of withstanding implantation or biological use.